Field of the Invention
The present invention relates to an electrically assisted steering system for a vehicle, and more particularly to a system and method for detecting failure of a steering angle sensor.
Related Prior Art
The existing steering wheels for vehicles are mostly provided with a SAS (steering angle sensor) to sense the rotation angle and speed of the steering wheel. In addition to acquiring the angle of the steering wheel rotated by the driver, the SAS also sends the acquired data to the ECU (electronic control unit), and the data is used as reference data for the steering system and steering operation.
The SAS is usually installed on the steering shaft of the wheel. When the steering shaft rotates with the steering wheel, rotation direction and angle will be detected by the SAS and inputted to ECU of the EPS (electric power steering) as one of the inputted signals. Then, EPS calculates required assistant power and direction based on the inputted signals regarding rotation torque and speed of the vehicle, and sends a control signal to the electric motor. Then, the motor generates the required rotation torque as an assistant power and sends it to the steering tie rod via the gear reduction mechanism and toothed wheel and rack.
ECU of the EPS acquires these data and calculates the difference between torque required for normal safe driving and the torque applied by the driver when the driver has the intention to make a turn. Then, the ECU gives orders to adjust the assistant power generated by the motor, which consequently corrects the torque required when the driver makes a turn, thus enhancing driving safety.
EPS must be equipped with the SAS, the SAS converts the rotation angle of the steering wheel into a signal indicating the direction that the driver intends to drive. EPS can tell the intention of the driver by calculating the rotation angle and the rate of change of the rotation angle. However, when a failure of the SAS occurs, the ECU of the EPS is unable to tell the turning direction of the vehicle, which negatively affects the handling of the steering wheel.
The technique disclosed in U.S. Pat. No. 8,849,498B2 and Chinese Pat. No. 100519301C is to check whether the rotation direction of the motor is the same as the rotation direction of the steering wheel, if so, it means that the rotation angle signal of the steering wheel works well. If not, it has to check whether the rotation direction of the motor is the same as the torque direction of the steering wheel, if so, go back to the initial step and recheck again, if they are still not the same, it means that the rotation angle of the steering wheel fails, and the failure signal will be shared with other control modules.
The abovementioned patents are able to check whether the rotation angle signal of the steering wheel fails or not, however, no matter the way of checking, they always use the direction as a decision criteria. The use of direction as the only decision criteria would lead to inaccurate determination since there are too much variables. Besides, the abovementioned patents only can check the failure of the rotation angle signal of the steering wheel, but fail to disclose a substitute technical means for the rotation angle signal of the steering wheel. Therefore, if the rotation angle signal of the steering wheel fails all of a sudden, it will cause the failure of the control system, and negatively affect the handling of the steering wheel.
JP 2014054890 employs the rotation angle of the steering wheel, the lateral swing rate of the vehicle, and the lateral acceleration as reference parameters to check if the SAS fails or not. If SAS fails, then it has to check, at different speeds of the vehicle, if torque and rotation angle signal of the steering wheel are within the predetermined range, and count the number of times of error occurrence. When the number of times of error occurrence reaches a predetermined value, it means the failure of the rotation angle signal of the steering wheel.
The above Japan patent also is able to check whether the rotation angle signal of the steering wheel fails or not, however, it requires more sensors to detect the lateral swing rate and the lateral acceleration, which inevitably increases the cost. Besides, in the existing electric assistant steering power system, the data of the lateral swing and lateral acceleration is normally acquired by external signals. In fact, these two kinds of signals cannot be acquired when the vehicle is moving, and as a result, the checking cannot be performed. Besides, the abovementioned Japan patent only can check the failure of the rotation angle signal of the steering wheel, but fails to disclose a substitute technical means for the rotation angle signal of the steering wheel. Therefore, if the rotation angle signal of the steering wheel fails out of a sudden, it will cause the failure of the control system, and negatively affect the handling of the steering wheel.
The present invention is aimed at solving the above problem, that is, when the rotation angle signal of the steering wheel fails, it won' cause the failure of the control system, and negatively affect the handling of the steering wheel.